Switching transistors used for this purpose must be driven with a defined current in order to keep the power demand and the losses of the driver stage low. Moreover, the switching transistor ought not to be operated at saturation since it then impairs the switching behaviour of the switching transistor as a result of high recombination times of electrons and electrons-holes in the transistor. Furthermore, the switching transistor ought, in each case in the shortest possible time, both to be turned off and to be turned on into a volume resistance which is as small as possible, in order to minimize the losses in the switching transistor. These partly conflicting requirements should be fulfilled as optimally as possible by a circuit for driving a switching transistor.
PCT document WO 99/39435 discloses a circuit intended for the purpose of avoiding undesired saturation of the switching transistor. Said circuit comprises a comparator circuit, which monitors the voltage across the current input of the switching transistor for the purpose of monitoring saturation by comparing said voltage with a reference voltage, thereby effecting regulation. The comparator circuit may be realized, e.g. by a simple transistor circuit, the reference voltage being applied to the control input of said transistor circuit, the current input of said transistor circuit being added to the drive signal of the driver stage, and the current output of said transistor circuit being connected to the voltage via the current input of the switching transistor.
Document “Technology and Service” of SGS entitled “Power Supply Application Manual”, July 1985, pages 217-221 discloses a circuit including a switching transistor of the type wherein a MOS-transistor is connected in series with the collector/emitter path of the switching transistor, so that the main current of the switching transistor flows via the series connection of the switching transistor and the MOS transistor. A switching voltage is fed to the gate electrode of said MOS transistor in order to periodically switch on and off the switching transistor and the MOS transistor synchronously. The advantage of said MOS-solution is that the emitter current being suddenly stopped and the current in the collector keeps on flowing through the base of the switching transistor until switch-off. This switch off characteristic is independent and no more function of the driver circuit providing said switching voltage. A negative source at the base is also no more necessary; the closed emitter assures the reverse voltage condition.
Within such a circuit there are special requirements to the switching voltage fed to the gate electrode of the MOS transistor. First said switching voltage must have a defined amplitude for properly switching on and switching off said MOS transistor. Secondly the source providing said switching voltage must have relatively low impedance. This is due to the fact that the MOS transistor has a relative high internal capacity at its gate electrode, which might unduly affect the shape of the switching voltage if the impedance of the source providing the switching voltage is too high.
According to prior art the switching voltage for the MOS transistor is provided by a dedicated integrated circuit yielding at one pin the switching voltage for the MOS transistor with the appropriate amplitude and low impedance of the voltage source for said switching voltage. That means a special circuit is needed for providing said switching voltage.